1. Field of the Invention
This invention relates to a catalyst component and a process for preparing said catalyst component. The catalyst component may be employed with or without a cocatalyst in the polymerization of olefins to polyolefins. The process for preparing the catalyst component comprises the fluoriding of a support and further reaction of the support with a vanadium compound to produce a novel catalyst component which allows the molecular weight distribution of the polyolefins to be controlled by controlling the amount of fluorination and the fluorination temperature.
2. Description of the Prior Art
The use of vanadium-based catalysts in the polymerization of olefins is well known. When unsupported, vanadium catalysts usually assume the form of an oil or gum, and tend to cause fouling of the polymerization reactor. As a consequence, unsupported vanadium-based catalysts are unsuitable for use in a slurry or gas phase reaction process.
Supported vanadium catalysts suffer from the fouling problem to a lesser degree. However supported vanadium catalysts suffer from a series of shortcomings. First, supported vanadium catalysts tend to produce polymers having too broad molecular weight distributions (MWD), with the polymer including a significant amount of low molecular weight oligomers. These oligomers, when present in polymers used in the manufacture of blow molded articles, produce an unacceptable amount of smoke.
It would be highly desirable to have a supported vanadium catalyst which could be used in a gas or slurry phase polymerization process to produce polymer of narrower MWD for use in blow molding resin. Similarly, narrow MWD's are desirable for resin applications such as injection molding or linear low density polyethylene (LLDPE) film production.
For other applications, such as the manufacture of high density polyethylene (HDPE) films, it would be desirable to produce a high molecular weight resin having a broad MWD. In yet other applications, such as wire and cable coatings manufacture, it is desirable for the resin to have a MWD of intermediate breadth.
It would thus be desirable to have a supported vanadium catalyst for the production of high molecular weight polyolefins having an easily and accurately controllable MWD ranging from broad to narrow, as desired, depending on the intended use of the resin product. A clear need exists in the industry for a supported vanadium catalyst which can be tailored to provide resins of a specific MWD over a wide molecular weight range.
A second shortcoming of supported vanadium catalysts is that these catalysts, when used in the polymerization of ethylene or in the copolymerization of ethylene with other 1-olefins, exhibit low activity in comparison to supported titanium-based catalysts. It would thus be desirable to have a supported vanadium catalyst which would have increased activity.
A third shortcoming of supported vanadium catalysts is their need for relatively high levels of hydrogen during polymerization to control resin molecular weight. It would be desirable to have a supported vanadium catalyst that would have increased response to hydrogen for affecting chain termination and controlling molecular weight.
Finally, supported vanadium catalysts require relatively high levels of comonomer to prepare medium and low density resins. It would be desirable to have a supported vanadium catalyst which would have increased response to comonomer to effect resin density reduction. Good comonomer utilization means that less comonomer is needed to give the target resin density. This is important for both slurry and gas phase polymerization since large amounts of comonomer lead to upsets in process conditions by increasing the solubilizing power of the slurry diluent or forming droplets in the gas phase.
It is well known that fluorided supports can be used to advantage with chromium catalysts. In that context, fluorination increases chromium catalyst activity, narrows resin MWD, but decreases resin melt indices. Decreases in resin melt index (MI) are indicative of increased molecular weight and decreased response of the catalyst to hydrogen.
Fluorination of chromium catalyst supports was disclosed in U.S. Pat. Nos. 2,825,721, and 2,951,816. U.S. Pat. No. 3,130,188 described the use of ammonium silicofluoride in conjunction with supports for chromium-based catalysts. U.S. Pat. No 4,011,382 discloses a titanated and fluorided support for a chromium catalyst and notes that increasing the fluorine content, while improving the rate of incorporation of comonomer, also decreases the resin melt index.
Similarly, U.S. Pat. No. 4,077,904 describes a fluorided silica support for a chromium catalyst. The data show that fluorination decreased resin melt index and had no effect on MIR, i.e. that the average molecular weight increased while the MWD distribution remained unchanged.
The earliest mention of a vanadium compound associated with fluorine for olefin polymerization is in U.S. Pat. No. 3,304,295 in connection with an unsupported catalyst mixture U.S. Pat. No. 4,262,105 discloses the fluorination of support material consisting of magnesium compounds for TiCl.sub.4 catalysts for the polymerization of ethylene. U.S. Pat. No. 3,936,431 covers fluorided alumina-silica and alumina-calcium oxide as supports. Similarly, U.S. Pat. No. 4,258,159 discloses fluorided alumina and magnesium chloride as supports for titanium alkoxide catalysts.
U.S. Pat. No. 4,359,403 teaches the addition of fluorine-containing compounds to silica prior to or during heating for the sole purpose of improving the activity of the silica supported catalyst. The catalyst is prepared by reacting the fluorided silica with a magnesium compound and then with the active metal. The patent also teaches the addition of alcohol for the purpose of improving the catalyst response to hydrogen. No mention is made of improved hydrogen response or comonomer response caused by the fluorination, nor is any mention made of the ability to control molecular weight distribution through independent control of the amount of fluorination and the fluorination temperature.